黏性土主动区临界深度$z_0$的朗金表达式为？

1. A

   $2c/(\gamma\sqrt{K_a})$

   正确
2. B

   $c/(\gamma K_a)$

3. C

   $2c\gamma/K_a$

4. D

   $c\sqrt{K_a}/\gamma$